Abstract

A functional testing system for programmable logic devices. Test vectors are generated by a shift register and applied to the contact pins of the logic device through isolation elements so that all pins may be treated alike regardless of whether they are inputs or outputs. The logic level on pins that are outputs are controlled by the logic device, while logic levels on pins that are inputs are controlled by the shift register. The response of the logic device to the test vector is recorded in an output shift register and the response is then shifted out of the shift register to one input of an exclusive OR gate that also receives outputs from predetermined stages of the test vector shift register to create a pseudo-random function. The output of the exclusive OR gate is shifted into the test vector shift register as each bit of the logic device's response is applied to the exclusive OR gate thereby creating a new test vector. The number of test vectors applied to the logic device is counted and when a predetermined number is reached, the test terminates and the current test vector is then stored and compared to the final test vector obtained by performing the same test on an identical circuit known to be operating correctly. The testing system thus evaluates the functionality of the logic device while also providing the stimulus to the device.

Description

DESCRIPTION

1. Technical Field

This invention relates to electronic test equipment, and more particularly to a system for automatically performing functional tests on digital logic devices.

2. Background Art

Programmable logic devices consist of arrays of logic elements that may be programmed by the user by selectively blowing fuses to implement complex logic functions. Programmable logic arrays are programmed in a manner similar to that of programmable read-only memories (PROMS), and they may be programmed with similar equipment. Despite the similarities, logic devices require different programming support functions than PROMs. When a PROM is programmed, the data that is stored in the PROM is usually a high-level program which either has been compiled into machine code or is an assembly language program that has been assembled into machine code. The binary information is then stored in read-only memory (ROM) for editing, and when the data is correct, the PROM is programmed by loading it with data. The data in the PROM is then checked for any programming errors.

Logic devices, on the other hand, are programmed using a different sequence of programming steps. The logic structure of a programmable logic array is first defined in the system design, as opposed to a computer program. The logic structure is then specified using some type of data preparation system. Once the logic structure specification has been input to the data preparation system, a fuse pattern for the device to be programmed is generated and used to program the device. Like PROMs, the programming of logic devices should be verified after programming. In order to test the functionality of programmable logic devices, it is necessary to implement some type of functional testing in which the device is exercised and its responses determined, such as by comparing the response to the response of a device known to be operating properly.

Functional tests of programmable logic devices are conventionally performed using various testing methodologies, depending upon whether combinatorial or sequential logic is involved. Combinatorial devices are the easiest devices to test since the outputs are a function of only the input conditions. Combinatorial devices are, therefore, very receptive to generalized testing algorithms. Sequential logic, on the other hand, imposes rigid requirements on a testing scheme because the outputs are a function of not only the current inputs but also the previous state of the device. Sequential devices are, therefore, not usually receptive to generalized testing algorithms.

One functional testing algorithm involves transition counting. In transition counting, the device inputs are subjected to a changing test pattern. As the pattern changes, transitions in the output occur; and these transitions are counted to verify that the correct number of transitions has occurred.

Logic simulation is another method of determining the functionality of programmable logic devices. To simulate the device, the device's logic structure must be defined on a computer; and the simulated device is then subjected to a test pattern. The output of the simulation will tell whether or not the device's logic structure is responding correctly to the input test pattern.

Another testing technique, known as "structure testing," involves the use of a test pattern that, for combinatorial devices, will consist of every possible input state for the device. For sequential devices, structure testing patterns are structured such that the device will go through all of its programmed state transitions.

All of the above testing techniques have certain limitations. Transition counting, for example, is limited to only certain types of errors. If more than one device output is defective, the error detection probability becomes significantly less than for single output errors. This is because a defective device can cause the correct number of transitions to occur even though two device outputs are defective. Structure tests and simulation require large amounts of time to generate all of the possible input states. This is particularly inefficient since, for most logic devices, only a small portion of the possible input states actually occur in use.

DISCLOSURE OF INVENTION

It is an object of the invention to provide a system that can automatically test for the proper functioning of a wide variety of logic devices, including progammable logic devices.

It is another object of the invention to provide a system that is capable of testing the functionality of sequential logic devices as well as combinatorial devices.

It is still another object of the invention to provide a functional testing system for logic devices that can be set up for testing without manually generating a detailed test procedure based upon a detailed knowledge of the device's function.

It is a further object of the invention to provide a functional testing system for logic devices that has a high probability of detecting all types of errors including those exhibiting the same number of transitions as a properly functioning device.

These and other objects of the invention are provided by a system for testing logic devices having a plurality of electrical contacts. Although the system is primarily suited for testing programmable logic devices, it is also capable of testing virtually any logic device that generates specific outputs in response to various combinations of inputs. The functional testing system includes a multi-stage shift register generating a test vector that is applied to the logic device under test through respective isolation elements. As a result, all contact pins of the logic device can be treated alike regardless of whether they are inputs or outputs. The logic levels on pins serving as inputs are controlled by the corresponding shift register stage and the logic level on contact pins serving as outputs are controlled by the logic device. After the logic device has responded to the test vector in the shift register, a new test vector is created from the response. The new test vector is created by applying the outputs of several shift register stages to an exclusive OR-gate along with at least some of the contact pins of the logic device. The output of the exclusive OR-gate is applied to the input of the first shift register stage. The shift register stages applied to the exclusive OR-gate are selected to maximize the pseudo-random nature of the new test vector. Although the contact pins of the logic device can be applied simultaneously to the exclusive OR-gate, they can also be sequentially applied through a multiplexer, thereby reducing the number of necessary inputs to the exclusive OR-gate, but increasing the amount of time required to create the new test vector. Where a multiplexer is used in this manner, the logic device does not respond to the contents of the shift register until the new test vector is created by sequentially applying a predetermined number of contact pins to the exclusive OR-gate while shifting the output of the exclusive OR-gate into the shift register. The logic device then responds to the contents of the shift register while the shift register is prevented from shifting data from one stage to the next. Each cycle of the test thus consists of a stage in which the new test vector is built and a stage in which the logic device under test responds to the new test vector. Each such cycle is counted and when a predetermined number of cycles have been completed, the final test vector in the shift register is read out and compared with the test vector that was in the shift register when the same test was performed on another logic device identical to the device under test and known to be functioning properly. While all contact pins of the device under test may be driven by the shift register, provision is also made for treating certain pins such as the enable and clock inputs separately so that the logic device is not pseudo-randomly clocked and enabled. The system thus not only determines the functionality of the device under test, but it also provides the stimulus to the device.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the basic concept of the functional testing system.

FIG. 2 is a block diagram of one embodiment of the testing system.

FIG. 3 is a schematic of the memory subsystem of the functional testing system that generates enabling and control signals from addresses of the programmer and outputs the final test vector to the programmer.

FIG. 4 is a schematic of the controller subsystem of the functional testing system which provides control, clock and enable signals to the device under test and determines when the test should be terminated.

FIG. 5 is a logic diagram for a programmable logic device used in the controller subsystem of FIG. 4 to provide various control signals for the functional testing system.

FIG. 6, comprised of FIGS. 6A and 6B, is a schematic of the sink driver subsystem of the functional testing system that contains a shift register and exclusive OR-gate creating the test vector and circuitry for applying the test vector to the device under test through isolation elements.

FIG. 7, comprised of FIGS. 7A and 7B, is a schematic of the waveform generator subsystem of the functional testing system which generates various analog voltages that are indirectly applied to the device under test for programming the device.

FIG. 8 is a schematic of the rise-time subsystem of the functional testing system for applying the analog voltages generated by the wave form generator of FIG. 8 to the device under test with a predetermined rise time.

FIG. 9 is a schematic of the comparator subsystem of the functional testing system that determines the logic level on each contact pin of the device under test as it responds to the test vector and stores the results of each comparison in a shift register.

FIG. 10, comprised of FIGS. 10A and 10B, is a schematic of the pin driver subsystem of the functional testing system that applies logic levels to the device under test for programming the device.

FIG. 11 is a schematic of a testing and programming adapter that interfaces the device under test to the remainder of the functional testing system in order to adapt the testing system to the specific type of logic device being tested.

BEST MODE FOR CARRYING OUT THE INVENTION

The basic system concept of the testing device 10 of the present invention for testing digital logic devices is illustrated in FIG. 1. Each input/output pin of a programmable logic device under test 12 is connected to a corresponding output of a shift register 14 through a respective isolation element, such as a resistor 16. The outputs of certain shift registers are also applied to an exclusive OR gate 18, and the output of the gate is applied to the input of the shift register 14. A multiplexer 20 sequentially connects each of the pins of the device under test 12 to a remaining input of the exclusive OR gate 18. The shift register 14 and multiplexer 20 are driven by a timing and control logic circuit 22 so that the data shifts through the shift register in synchronism with the switching of each pin of the device under test 12 to the exclusive OR gate 18. The shift register 14 thus provides the stimulous for the device under test 12 as well as a means for detecting an incorrect response thereto.

The shift register 14 treats all datacarrying pins of the device under test 12 the same, regardless of whether they are inputs or outputs. The shift register 14 thus generates the input signals to the device under test 12. In the event that a pin of the device under test 12 is an output instead of an input, the low impedance at the output of the device under test 12 controls the voltage level on the pin. The state of the shift register 14 is a pseudo-random sequence. This sequence depends not only upon the preset initial state of the shift register, but also, all states of the shift register 14 occurring thereafter. These states, in turn, are a function of the programming in the device under test 12. By properly choosing certain outputs of the shift register 14 to be applied to the exclusive OR gate 18 in accordance with known mathematical techniques, the pseudo-random nature of the shift register 14 can be maximized, providing a more complete stimulus to the device under test 12.

The primary advantage of treating all pins of the device under test 12 alike, without regard to whether they are inputs or outputs, is that the testing is simplified since the function of the device need not be known. The device under test 12 is merely connected to the shift register 14, the shift register 14 is preset to an initial state, and a given number of clock pulses are applied to the shift register 14 and multiplexer 20. The resulting condition of the shift register 14 or "signature" is then compared to the signature obtained from the shift register 14 when a device under test 12 known to be functioning properly has undergone the testing procedure in the identical manner. The system 10 can, therefore, be set up to test a programmable logic device 12 quickly and easily since it is not necessary to manually generate a detailed test procedure based upon the function of the device 12.

Instead of feeding the pins of the device under test 12 to the exclusive OR gate 18 with the multiplexer 20, all of the pins of the device under test 12 could be directly connected to respective inputs of the exclusive OR gate 18. The advantage of the multiplexer 20 is that the size of the exclusive OR gate 18 can be reduced. Conversely, the disadvantage of the multiplexer 20 is that in order to test all of the pins of the device under test 12, the multiplexer must apply each of the pins to the shift register 14 in sequence before a new set of inputs is applied to the device under test. Thus, the test requires a relatively longer time to complete.

One embodiment of a testing device 10 implementing the system concept illustrated in FIG. 1 is shown in the block diagram of FIG. 2. The testing device 10 is in the form of an add-on circuit to a conventional PROM programmer 30, such as a Model 29-A or a 100-A universal programmer sold by Data I/O Corporation of Redmond, Wash. The universal programmer 30 is a microprocessor-based device that includes a data bus 30a, an address bus 30b, a read/write line 30c, and an internal clock (not shown) with a clock line 30d. The testing device 10 further includes a controller/memory board 32 that generates timing and control signals and decodes the address bus 30b to generate various enabling signals, contains software common to all programmable logic devices to be tested, and facilitates the reading of the signature from the shift register 14. Also provided are a waveform generator 34, which generates software-controlled analog voltages that are applied to the device under test 12; a rise-time comparator 36, which generates signals having software-controlled rise time for programming the programmable logic device under test 12 prior to testing and provides comparison of signal levels of the device under test 12 to predetermined values; a sink driver 38, which includes the shift register 14 (FIG. 1), the exclusive OR gate 18, and circuitry for applying the outputs of the shift register 14 to respective pins of the device under test 12; a pin driver 40 for applying logic levels from the waveform generator 34 to program the programmable logic device under test 12; and an adaptor module 42. The adaptor module 42 contains software that is specific to the particular programmable logic device being programmed and tested, and it is the unit that physically receives the device under test 12. Each of these subsystems described above is explained in detail below.

A schematic of a memory portion of the controller/memory board 32 is illustrated in FIG. 3. The address bus 30b of the programmer 30 includes sixteen address bits. For each of 216 states of the address bus 30b, predetermined functions must occur in the system. As a result, the sixteen address bits are decoded to provide a number of enabling or control signals. The basic concept behind this decoding is to perform a primary decode in which a four-bit control word and several enable signals are generated. The control word is applied to several secondary decoder circuits in parallel, each of which is enabled by its individual enable signal. In this manner, a large number of control signals are generated to implement certain functions on the basis of selected addresses of 216 possible addresses. The twelve high-order bits of the address bus 30b are applied to a pair of primary decoder PROMs 50,52, each of which generates predetermined combinations of four-bit outputs for each combination of input or address. As mentioned above, the decoder PROMs 50,52 generate enable signals for other portions of the circuit, depending upon the particular address received from the programmer 30. The high-order outputs from the decoder PROM 52 are used as high-order address bits for the decoder PROM 50. An OR gate 54 is used to generate an extra output bit for certain addresses in which either the Q1 output of decoder PROM 50 or the Q1 output of decoder PROM 52 is high.

The outputs of PROM 50 for various addresses are as shown in the following truth table:

The four lowest order address bits, two outputs from the decoder PROM 50, one output from decoder PROM 52, and an output from the OR gate 54 are applied to a primary decoder latch 56. The decoder latch 56 is clocked by a clock signal V02 of the internal clock of the programmer 30 carried on the clock line 30d so that it thereafter generates an eight-bit word determined by the address on the address bus 30b of the programmer 30 and the data entered into the decoder PROMs 50,52. Basically, the decoder latch 56 synchronizes the address bus 30b to the clock signal V02. Four bits A1'-A4' from the decoder latch 56 form the control word mentioned above and are applied to various secondary decoders, as explained in greater detail below. The three remaining bits C1-C3 from the decode latch 56 are used to enable these secondary decoders. One such secondary decoder 58 receives three low-order bits of the control word from the decode latch 56 and is enabled by the Q3 output of the decode latch 56 to generate appropriate signals during a pulse of the clock signal V02.

The Q2 output of the decoder PROM 50 and the Q0 and Q7 outputs of the decoder latch 56 are applied to respective OR gates 60,62,64, which have their outputs synchronized to the clock signal V02 of the programmer 30. The OR gate 60 generates a GATE ENABLE signal, the OR gate 64 generates a C3 signal for enabling other secondary decoders, and the OR gate 62 enables an internal data gate, as explained below. The GATE ENABLE signal is applied to the programmer 30 to allow the programmer 30 to input or output data from its data bus since the data bus is also used for functions internal to the programmer 30.

Thirteen bits of the address bus 30b are also applied to a PROM 70, which generates an eight-bit word, depending upon the software contained therein and the particular address applied to the PROM 70. The software in the PROM 70 is common to all programmable logic devices of the type being tested, and is generated whenever the Q0 output of the decoder latch 56 goes low. At that time, a data gate 71 is enabled by the OR gate 62 so that the eight-bit word from the PROM 70 is applied to circuitry on the controller portion of the controller/memory board 32 described below. (FIG. 4). The object code for data stored in the PROM 70 in Motorola Exorciser Format, Code 82, is as follows: ##SPC1## // // //

The remaining circuitry illustrated in FIG. 3 is a read register 72 that is used to apply the bits of the shift register 14 (FIG. 1) to the data bus 30a of the programmer 30. Basically, the read register 72 sequentially receives bits from one shift register output. After eight bits have been stored in the read register 72, they are shifted out to the data bus 30a of the programmer 30 in parallel; and eight more bits are then stored in the same manner. The shift register is clocked at its CP input by a master oscillator (described below) and it shifts in either direction, depending upon the signal applied for its S inputs from circuitry in the controller portion of the controller/memory board.

The controller portion of the controller/memory board 32 is illustrated in FIG. 4. The controller portion includes a master oscillator 80 of conventional design generating a clock pulse output CP through an inverter 82. The master oscillator 80 also clocks a control counter 84 through an inverter 86, which, in turn, drives a second control counter 88. Basically, the counters 84,88 count to 48 and generate an eight-bit word corresponding thereto. The shift register 14 (FIG. 1) is clocked for the first 32 bits, and each of the 32 pins of the device under test 12 are applied to the shift register in sequence. At the end of 32 bits, a new signature or "vector" has been generated. Thereafter, for the next sixteen bits, the shift register 14 is not clocked; but the device under test is exercised to generate an output responsive to the new vector and this output is then stored by the system.

The control counter 88 clocks a vector counter 90, which counts the number of 48-bit periods in which a new vector is generated. When 128,000 vectors have been generated, the Q14 output of the vector counter 90 goes high and halts further testing. At that time, the final vector in the shift register 14 will be a function of the response of the device under test 12 to all 128,000 vectors. The probability is exceedingly small that a correctly functioning device and an incorrectly functioning device will have the same vector at the end of the 128,000 vector test. This 128,000 vector test cycle can be extended to include additional cycles at the user's option without initializing the register 14 between cycles. It is to be understood that while a 128,000 vector test cycle is described herein, other cycles of sufficient length may be used.

The outputs of the control counters 84,88 are applied to the inputs of a test PROM 92 and a controller 94, which is a programmable logic array. The test PROM 92 and the controller 94 also receive the outputs of a mode register 96. Basically, the test PROM 92 provides the device under test with clock and enable signals for each of the sixteen bits in which the device is responding to a new vector. The particular combination of clock and enable signals for each of the sixteen bits, as well as their timing, is controlled by a four-bit word from the mode register 96. These clock and enable signals for the various outputs of the mode register 96 and counter 84 are as shown in the following truth table for the PROM 92 test:

The mode register 96 is controlled by the programming in the PROM 70 (FIG. 3) and the particular address from the programmer 30 (FIG. 1). In this manner, the system is, in effect, adapted to the particular characteristics of programmable logic devices. The mode register 96 is clocked by a C6 bit from a secondary decoder in the wave form generator 34, as explained in greater detail below. The output of the test PROM 92 is clocked into a control register 100 by the master oscillator 80 through a synchronization register 85.

The controller 94 provides certain control signals throughout a 128,000-vector test. It receives a TEST input bit from the control register 100 which is high for the sixteen clocks that the device under test 12 is responding to a new vector. It also receives a RUN input bit from the control register 100 that is high at all times until 128,000 vectors in a test have been generated. At that time, the RUN bit goes low, thereby clearing the counters 84,88,90 in preparation for a new test. The other inputs to the controller 94 are a four-bit word from the mode register 96 indicative of certain characteristics of the test to be performed and a five-bit word from the counters 84,88,90. The controller 94 is a programmable logic device programmed as shown in the logic diagram of FIG. 5. The controller 94 is a PAL, part no. DMPAL 16L8CP made by National Semiconductor, Santa Clara, Calif.

During the sixteen-clock test period for the final vector, the TEST input and the Q14 output of vector counter 90 are both high. These two high bits are applied to a status AND gate 110, thereby generating a high as a STATUS signal that is applied to the data bus 30a of the programmer 30 (as explained below) in order to inform the programmer 30 that the test is completed.

The TEST bit is also inverted by an inverter 112 and applied to a chip-enable AND gate 114, which is connected to the test PROM 92. The AND gate 114 also receives an input from an AND gate 116, which receives two bits from the mode register 96. In certain modes, the output of the AND gate 114 will thus be high during the sixteen-bit period that the device under test 12 is responding to a new vector. Certain programmable logic devices have dedicated enable and clock pins; and if these pins were randomly driven, the device would be randomly enabled and clocked. Where it is desired that the device be continuously enabled and periodically clocked during the sixteen-bit test period, a mode is selected through the programmer 30 that causes the AND gate 114 to go high during the sixteen-bit test period. This high output causes the test PROM 92 to generate output bits CP1, CP2, CE1, and CE2 through the control register 100 and four respective inverters 118 which each have their outputs pulled high through pull-up resistors 120. As explained in greater detail below, these outputs allow certain pins of the device under test 12 to assume predetermined logic levels without regard to the levels applied to those pins by the shift register 14.

The controller 94 also generates an input register control (IRC) bit for causing the shift register 14 to respond to a clock pulse by either loading data into the shift register 14 in parallel or shifting each bit of the register. Similarly, the controller 94 also generates a shift/load (S/L) bit for performing the same function in a shift register for recording the response of the device under test 12 to a new vector, as explained in greater detail below. A RCIR bit is also generated by the controller 94 and applied to the shift register 14. When high, the RCIR bit disables the exclusive OR gate 18 (see FIG. 1) so that data recirculates in the shift register 14 without change. In this manner, data is transferred from the shift register 14 to the read register 72 (FIG. 3) which applied the data to the data bus of the programmer 30.

Finally, the controller 94 generates through a pair of inverters 122,124 shift signals for the read register 72 (FIG. 3), as mentioned above, and an input to the control register 100 through an inverter 126.

The test begins with the C6 input to the counter 84, a flip-flop, goes high. On the next leading edge of the clock pulse CP from master oscillator 80, this high is clocked to the corresponding output of the control register 100, thereby clearing the control counters 84,88 and the vector counter 90 through an inverter 130, and making the RUN input to the controller 94 high. Thereafter, the control counters 84,88 continue incrementing, causing the test PROM 92 and controller 94 to generate appropriate outputs until the vector counter 90 reaches 128,000, at which time it applies a low to the status AND gate 110.

The sink driver 38, as illustrated in FIG. 6, includes the shift register 14 (FIG. 1) in the form of a series of eight four-bit shift registers 150-156 arranged in correspondingly numbered pairs (indicated by the letters "A" and "B") to generate eight bits. The shift registers 150-156 are all clocked at the same time by the clock pulse CP through an inverter 158. The shift registers 150-156 may operate in either of two modes, a shift mode or a load mode. In the shift mode, selected by the IRC input being made high, data from an AND gate 159 is applied to the DS input of the shift register 152B and is sequentially shifted through its four stages before being shifted to shift register 152A and each of the other shift registers in sequence. In the load mode, selected by the IRC input being made low, each pair of the shift registers are selected by a secondary decoder 164 through inverters 166. At the clock pulse CP, data on the data bus 30a of the programmer 30 is shifted into the selected pair of shift registers, with the four low-order bits being shifted into the "A" shift registers and the high-order bits being shifted into the "B" shift registers. The initial state of the shift registers 150-156 can thus be preset prior to a test.

In the shift mode, the outputs of the shift registers 150-156 are applied to the device under test 12, as explained hereinafter, and each pin outputting the device's response to the vector is sequentially applied to another input of a parity generator 170 from the OUTPUT S0 line: the parity generator 170 functions as the exclusive OR gate 18 (FIG. 1) when the RCIR input is low. Five of its inputs are connected to the shift registers 150-156 stages 32, 23, 3, 2, 1 and its output is fed back to the input of the shift register 152B stage 32 through the NAND gate 159 in order to maximize the pseudorandom nature of the test vector. When the RCIR input goes high, a NAND gate 172 is disabled through an inverter 174, and the AND gate 159 functions as an inverter. In a recirculating mode, data merely recirculates through the shift register, allowing the data to be read by the read register 72 (FIG. 3) via the VECTOR S0 output. The outputs of the shift registers 150-156 are applied to respective inverters 180, which are, in turn, connected to the device under test 12 through respective resistors 182. Pull-up resistors 184 are provided since the logic "1" condition of the inverters 180 is an open circuit. Although most of the outputs of the shift registers 150-156 are applied directly to their respective drivers 180, four of these outputs are applied to four NAND gates 190-196. The NAND gates 190-192 act as inverters and apply respective bits of the shift register 14 to the chip-enable pins of the device under test 12. It will be noted, however, that the chip-enable pins may be overridden by the CE1 and CE2 inputs which are generated by the test PROM 92 (FIG. 4) in order to keep the device under test 12 continuously enabled. The NAND gates 194,196 gate inverted bits of the shift register 14 when they are enabled by the CP1EN and CP2EN inputs, respectively. When they are not enabled, the CP1 and CP2 inputs are routed to the clock pins of the device under test 12.

The waveform generator 34, as illustrated in FIG. 7, provides various analog voltages for programming the programmable logic device by burning in internal fuses and checking the characteristics of the fuses. It also supplies these voltages for testing the results of the programming by verifying the blown fuses, and it supplies power to the programmable logic device while being tested. Various control voltages are generated by a secondary decoder 200, which is enabled by the C1 bit from the primary decoder latch 56 (FIG. 3) and clocked by the master oscillator 80 in the controller portion of the controller/memory board 32, as illustrated in FIG. 4. The secondary decoder 200 may also be enabled by the A4' bit through an inverter 202. The particular outputs generated by the secondary decoder 200 are selected by the A1'-A3' inputs to the decoder 200, with the decoder 200 generating bits at its Y0 -Y7 outputs in accordance therewith. The operation of the secondary decoder 200 is synchronized to clock V02 of the programmer 30. The values for the analog voltages are indicated by an eight-bit word on the data bus 30a and applied to a data latch 204, triggered by the programmer clock bit V02 through an inverter 206. The outputs of the data latch 204 are pulled high through pull-up resistors 208 and are applied to the inputs of four digital-to-analog converters 210-216, which are individually enabled by respective outputs from the secondary decoder 200. These outputs are held high through respective pull-up resistors 218-224. The generated analog voltages of the converters 210-216 are connected to respective operational amplifiers 226-232, and the outputs of the operational amplifiers are applied through respective resistors 234-240 to respective amplifiers 242-248.

The amplifier 242 drives the gate of a field-effect transistor 250 which has a pair of current-sensing resistors 252,254 connected between a power supply and the drain. The source of the field-effect transistor 20 generates the power supply voltage VCC for the device under test through diode 256, and this voltage is fed back to amplifier 242 through a zener diode 258, a diode 260, a potentiometer 262, and a resistor 264. Adjustment of the potentiometer 262 controls the degree of negative feedback provided to the amplifier 242 and hence the magnitude of the power supply voltage VCC. The power supply voltage VCC is also low-pass filtered by a capacitor 268.

The voltage across the resistor 252 is indicative of the current through the field-effect transistor 250, and is applied through a resistor 270 to the base of a transistor 272. When the current-induced voltage across the resistor 252 exceeds a predetermined voltage (in the illustrated embodiment, about 0.6 of a volt), the transistor 272 becomes conductive and current is applied through a resistor 274 to the base of a transistor 276, which is normally held low through a resistor 278 and filtered by a capacitor 280. As explained below, the transistor 276 becomes conductive in the event of certain abnormal conditions in order to reset various circuits as explained below.

The current flowing through the field-effect transistor 250 is also indicated by the voltage across the series combination of resistors 252 and 254. This voltage is applied to the base of a transistor 282 through a resistor 284. Insofar as the voltage across the series combination of the resistors 252 and 254 will always be greater than the voltage across the resistor 252 alone, the transistor 282 can be expected to turn on before the transistor 272. Thus, the transistor 282, which is connected to the base of transistor 276 through a resistor 286 and a diode 288, will normally turn on the transistor 276 before it is turned on by transistor 272. However, the transistor 282 is prevented from turning on the transistor 276 by clamping the anode of the diode 288 to the voltage drop across one diode on occurrence of a low output of an inverter 290, which is connected to the anode of the diode 288 through a diode 292. Consequently, the transistor 276 is turned on by the transistor 272 after a sufficiently large current is flowing through the field-effect transistor 250. When the output of the inverter 290 is high, the transistor 282 will conduct at a relatively low current, thereby turning on the transistor 276. The inverter 290 is controlled by the outputs of a control register 296, which is clocked by the secondary decoder 200 and controlled by the output of the data latch 204.

In a manner similar to the circuit connected to the amplifier 242, the amplifier 244 drives the base of a transistor 300, which outputs at its emitter a chip-enable supply (CE SUPPLY) voltage. This CE SUPPLY voltage is fed back to the inverting input of the amplifier 244 through a diode 302, a potentiometer 304, and the parallel combination of a resistor 306 and a capacitor 308 which acts as a low-pass filter. The cathode of diode 302 is biased low through a resistor 310. The magnitude of the CE SUPPLY voltage may thus be adjusted by adjusting the potentiometer 304. The current through the transistor 300 is measured by a current-sensing circuit, indicated generally by reference numeral 311, which operates in the same manner as the current-sensing circuit for the field-effect transistor 250 described below. This circuit also turns on the transistor 276 when either of two current thresholds are exceeded, depending upon the condition of an inverter 312 which is controlled by the control register 296. Similarly, the output of the amplifier 246 generates a BIT SUPPLY voltage and turns on the transistor 276 through a current-sensing circuit 316 identical to the current-sensing circuit 311.

The collector of transistor 276, which is biased high through a resistor 320, goes low when the selected high or low threshold is exceeded for any of the current-sensing circuits described above, and the transistor turns on. When the collector of transistor 276 goes low, a reset memory flip-flop 324 is cleared, thereby causing the flip-flop 324 to generate a high at its Q output. This high turns on a transistor 326 through a resistor 328, thereby discharging a capacitor 330 and pulling a reference voltage (VREF) applied to the digital-to-analog converters 210 216 to zero, resulting in amplifiers 242248 being powered down. This reduced voltage reference causes the COMPARISON REFERENCE (COMP REF) output voltage generated by amplifier 248 to also be reset to zero. Thereafter, a high is applied to the data input of the flip-flop 324 from the data latch 204, and the clock input is released by the secondary decoder 200 and pulled high through a resistor 334. The Q output of the flip-flop 324 then goes low, turning off the transistor 326 and allowing the capacitor 330 to charge through a resistor 336, thereby restoring the original reference voltage to the digital-to-analog converter 210 216. This reference voltage may be adjusted by adjusting potentiometer 338, which is connected to a voltage reference zener diode 340. The flip-flop 324 also generates a RESET signal through an inverter 342.

The flip-flop 324 is also cleared by transistor 344 and the RESET signal generated when power is initially applied to the system. When a power is initially applied, the clear (CL) input of a flip-flop 346 is initially held low through a capacitor 348, thereby causing the Q output of the flip-flop 346 to go high. This high is applied through a resistor 350 to the base of the transistor 344, thereby pulling the clear (CL) input to the flip-flop 324 low and resetting it. The capacitor 348 then begins charging through a resistor 352. When the capacitor 348 charges to a voltage exceeding the threshold voltage of the CL input of the flip-flop 346, the output of the flip-flop 346 is clocked low on the next clock pulse V02, thus turning off the transistor 344 and removing the low from the CL input of the flipflop 324. In the event that power is lost for even a short period of time, the capacitor 348 is quickly discharged through a diode 354, thereby once again resetting the flipflop 324 for a short period of time until the capacitor 348 is once again charged.

The rise-time circuit 36, as illustrated in FIG. 8, provides output waveforms having predetermined rise times for programming the device under test 12. The rise times for these waveforms are controlled by an eight-bit word on the data bus 30a. The eight-bit word is applied to a switch control register 400 cleared by the reset from the flip-flop 324 (FIG. 4) and clocked by the C5 bit of the secondary decoder 200 (FIG. 6). As explained in greater detail hereinafter, the switch control register 400 generates outputs to control the rise times of the CE SUPPLY and BIT SUPPLY outputs from the waveform generator of FIG. 7.

The first switch circuit is formed by a pair of transistors 402 and 404 powered by the BIT SUPPLY signal. These transistors 402,404 output a bit switch number one (BIT SW #1) signal having a rise time selected from one of three values. The switch is turned off by a low applied to a NAND gate 406, which allows current to flow through a resistor 408 to turn on a transistor 410. The transistor 410 then pulls the collector of the transistor 402 to ground, causing the emitter of transistor 404 to follow. When the input to the NAND gate 406 goes high, the base of the transistor 410 goes low, thereby turning off transistor 410 and allowing a capacitor 412 to charge with a current in proportion to the voltage across a resistor 414. The transistor 402 thus acts as a current source for the capacitor 412 so the rise time of the BIT SW #1 signal is proportionate to the time constant of the combination of resistor 414 and capacitor 412. The time constant can vary by allowing a capacitor 416 to also charge, in which case a transistor 418 is turned on by applying a high output of the switch control register 400 through a resistor 420 to the base of the transistor 418. Similarly, a capacitor 422 may be placed in parallel with the capacitors 412,416 by turning on a transistor 424 by applying a high output of the switch control register 400 through a resistor 426 to the base of the transistor 424.

In a similar manner, a second switch circuit, indicated generally by reference numeral 430, outputs a bit switch number two (BIT SW #2) signal, except that the rise time may be only one of two values, instead of three (as with the BIT SW #1 described above). Finally, a third switch circuit, indicated generally by reference numeral 432, outputs a chip-enable switch (CE SW) signal having a rise time that may be selected as one of three values. The CE SW signal is generated from the CE SUPPLY output of the waveform generator (FIG. 7).

Voltage cutout circuitry is provided to disable all three of the switch circuits described above whenever power is reduced or removed from the testing device 10. Accordingly, the power supply voltage is applied to a transistor 440 through a voltage dropping zener diode 442 and resistor 444. When the power supply voltage drops, the base of the transistor 440 is pulled to ground through a resistor 446, thereby cutting off transistor 440 and allowing the bases of the transistor 402 and a pair of transistors 431 and 433 to be pulled to the power supply voltage through a potentiometer 448. The transistors 402,431,433 are thus turned off, thereby allowing the bases of their respective emitter follower output transistors to fall to zero volts. When the power supply voltage is sufficient to turn on the transistor 440, the bases of the transistors 402,431,433 are turned on when sufficient current is flowing through resistor 450, connecting the transistor 440 to the potentiometer 448. The voltage cutout circuitry described above is provided to remove power from the device under test 12 during a programming operation if the power supply voltage of the testing device 10 starts to drop. Without such circuitry, the fuses of the device under test 12 could be blown at random, thereby destroying the usefulness of the device.

The CE SW BIT #1 and BIT SW #2 signals, as well as a VCC SENSE signal from the circuitry generating the power supply voltage VCC (FIG. 7), are monitored by respective comparators, indicated generally by reference numeral 460. Accordingly, each of these monitored signals is connected to respective pairs of voltage divider resistors, indicated generally by reference numerals 462,464, the center of each pair being connected to the positive input of the respective comparator 460. A positive voltage applied through pull-up resistors, indicated generally by reference numeral 466, prevents the voltage applied to any of the voltage divider resistors 462,464 from being pulled below the power supply voltage. The negative inputs of the comparators 460 are connected to the comparison reference (COMP REF) voltage that is generated by the waveform generator 34 (FIG. 7). Thus, the output of each comparator 460 indicates the logic level of the respective signal that it monitors. The outputs of comparators 460 are biased high through pull-up resistors, indicated generally by reference numeral 468, and are applied to a status gate 470 that is enabled by the C4 bit from the secondary decoder 58 (FIG. 3). The status gate 470 also receives the STATUS signal from the status AND gate 110 (FIG. 4), which indicates that 128,000 test vectors have been generated and the test is now completed. All of the outputs of the status gate 470 are applied to the data bus 30a of the programmer 30.

It will be remembered that the test vector is applied to the device under test 12 by the sink driver 38 (FIG. 7). Basically, as described in greater detail above, the test vector is generated by the shift register 14. Each bit of the shift register is applied to the device under test 12 through the resistor 16 or other isolation element. Thus, pins of the device under test 12 that are inputs will have the same logic level as the corresponding bit output by the shift register 14. In contrast, pins of the device under test 12 that are outputs will have a logic level determined by the device under test rather than the corresponding bit of the shift register 14.

In order to determine the logic levels on the pins of the device under test 12, the logic levels on the pins are stored by a 32-bit shift register 500 in the rise-time/comparator 36, as illustrated in FIG. 9. The shift register 500 consists of four eight-bit stages 502-508. Each of the 32 pins of the device under test 12 is applied to the respective positive inputs of comparators, indicated generally by reference numeral 510. The negative inputs of the comparators 510 are connected to the comparison reference (COMP REF) voltage generated by the waveform generator 34 (FIG. 7). The amplitude of the COMP REF voltage is determined by the digital-to-analog converter 216 (FIG. 7), which is, in turn, controlled by the programmer 30. The programmer 30 thus determines the logic level, differentiating a logic "0" from a logic "1." The outputs of the comparator 510 are each applied to an input of a respective bit of the 32-bit shift register 500, all of which are biased high through respective pull-up resistors, indicated generally by reference numeral 512.

As described above, after a vector has been created, the device under test 12 is exercised for sixteen bits, during which time the response of the device under test 12 to the vector is present on the pins of the device under test. The shift load (S/L) bit generated by the controller 94 (FIG. 4) is then made low, and the shift register 500 is clocked by the clock pulse output CP from the master oscillator 80 (FIG. 4), thereby causing the comparison of the voltages on all 32 pins of the device under test 12 with the comparison threshold voltage to be written into the shift register 500 in parallel.

A new test vector is then generated by the S/L bit going high and then clocking the shift register 500 with 32 clock pulses CP from the master oscillator 80. Each bit of the shift register 500 is then sequentially presented to the OUTPUT SO line and used to perform two functions. First, the bits are applied to the parity generator 170 (FIG. 6b), which as described above, acts as an exclusive OR gate. Thus, the shift register 500 applies a logic level corresponding to each pin of the device under test 12 to the exclusive OR gate in sequence for 32 clock pulses in order to build a new test vector. Under these circumstances, the shift register 500 functions as the multiplexer 20 of FIG. 1. Each bit of the shift register 500 is also applied from the OUTPUT SO line to the read register 72 (FIG. 3). After eight bits have been shifted into the read register 72, these eight bits are shifted out in parallel to the data bus 30a of the programmer 30. The programmer 30 can thus read the bits on the device under test 12. This function is not used for the pseudo-random testing technique, but rather to verify the fuse pattern in the device under test 12, if desired.

The system for testing programmable logic devices utilizing the inventive pseudo-random testing technique can also be used to program programmable logic devices. Accordingly, as illustrated in FIG. 10, programming data from the data bus 30a of the programmer 30 is clocked into a data latch 530 by the clock signal V02 of the programmer 30 applied through an inverter 532. The output of the data latch 530 is applied in parallel to four data latches 534-540. Each of the data latches 534-540 is individually clocked by respective outputs of a secondary decoder 542. The secondary decoder 542 is enabled by the C1 bit from the decode latch 56 (FIG. 3) during one pulse of the clock signal V02. The particular latch of the data latches 534-540 that is enabled is determined by the A1'-A3' outputs of the decode latch 56. The bits to be applied to the first eight pins of the device under test 12 are first recorded by the data latch 530 and then written into the data latch 534. The bits for each subsequent eight pins of the device under test 12 are then sequentially applied from the data bus 30a of the programmer 30 through the data latch 530 to each of the data latches 536-540 until the logic levels for all 32 pins are stored in the data latches 534-540.

Each output of the data latches 534-540 is applied to a respective driver circuit, indicated generally by reference numeral 541. Each of the driver circuits 541 consists of a switching transistor 550 having its emitters connected to ground through a resistor 552. The collector of the switching transistor 550 is connected to the base of a current source transistor 554 having its collector connected to a corresponding pin of the device under test 12. The emitter of the transistor 554 is connected directly to a supply voltage which is either the BIT SW #1, the BIT SW #2, or the CE SW signal from the waveform generator 34 (FIG. 7). A resistor 556 is connected between the emitter and base of transistor 554 to pull the base high when the switching transistor 550 is turned off. A capacitor 558 is connected in parallel with the resistor 556 to control the response time of the driver circuit 541. Thus, programming data for all 32 pins is stored in the data latches 534-540, and, when the BIT SW #1, BIT SW #2, and CE SW signals are switched on, current is selectively applied to all 32 pins of the device under test 12 to program the device. The data latches 534-540 are cleared by the RESET signal from the waveform generator 34 any time one of the current-sensing circuits senses that one of the current thresholds is exceeded or upon initial application of power, as explained above with reference to FIG. 7. The RESET signal to clear the data latches 534-540 is applied through a NAND gate 570 having its output biased high through a pull-up resistor 572.

The various types of programmable logic devices are programmed in various manners. Thus, the programming data that is appropriate for one type of programmable logic device will not be proper for another. In order to adapt the testing device 10 to the specific device that is being programmed, an adapter plugs into a receptacle connected to an I/O bus 573 and a CPU bus 574, as illustrated in FIG. 11. The device under test 12 then plugs into the adapter. The adapter is programmed for the particular type of programmable logic device that it receives. Accordingly, as illustrated in FIG. 11, a PROM 580 is programmed with data indicative of the characteristics of the particular programmable logic device which its adapter is designed to receive. The programmer 30 addresses the PROM 580 and the resulting data is clocked through a data latch 582 to the data bus 30a of the programmer 30. In this manner, the programmer 30 is able to adapt itself to the particular characteristics of the device being programmed. Thereafter, it stores appropriate data bits in the data latches 534-540 (FIG. 10) for programming the device. The object code stored in the PROM 580 for an adapted used with signetics IfL type programmable logic devices is as follows (stated in Motorola Exorciser Format, Code 82: ##SPC2##

The testing and programming adapter is provided with two separate receptacles 584,586 for the programmable logic device, both of which are connected to pins mating with the corresponding pins of the I/O bus 573. The pins of the device under test 12 are clamped to either the BIT SUPPLY voltage through diodes 588 or to the CE SUPPLY voltage through diodes 590 in order to prevent excessive voltages from being applied to the device. The BIT SUPPLY voltage is applied to the cathodes of the diodes 588 through a pair of voltage dropping diodes 591, and the resulting voltage is applied across a voltage spike clamp circuit comprising a resistor 592 and a filter capacitor 594 in parallel. Similarly, the CE SUPPLY voltage is applied to the cathodes of the diodes 590 through a pair of voltage dropping diodes 596, and the resulting voltage is applied across a voltage spike clamp circuit comprising a resistor 598 and a filter capacitor 600 in parallel.

In the absence of power supply voltage VCC, a VCC pull-up transistor 610 is turned on to apply voltage to the device under test 12, to which the power supply voltage VCC is connected. The voltage applied is current limited by a resistor 614. If the device under test 12 is mistakenly put in backwards, the voltage on the input of the device will be less than 0.7 volt; and this is sensed by the programmer 30 through a VCC SENSE output. The programmer 30 will then inhibit the digital-to-analog converter 210 (FIG. 7) from causing the power supply voltage VCC to be generated and applied to the device under test 12 since this might damage the device. A diode 612 is provided to prevent the power supply voltage VCC, when applied, from forward biasing the collector base junction of transistor 610. A resistor 611 holds the transistor 610 at cutoff when the VPUP input in floating.

The clock pins of the receptacles 584,586 are connected to a 5-volt power supply by respective diodes 620,622 through respective resistors 624,626 to provide fast rise times for the CP1 and CP2 signals. A pair of light-emitting diodes 640,642 mounted adjacent to each of the receptacles 584,586 is selectively illuminated by the programmer 30 by current flowing through respective resistors 644,646 to identify which of the receptacles 584,586 is being programmed.

The function performed by the testing system 10 for various addresses and words on the data bus of the programmer are as follows:

Claims (15)

We claim:

1. A system for testing a logic device having a plurality of electrical contacts which serve as either inputs to or outputs from said logic device, said system comprising:

a first shift register having a plurality of stages each receiving a respective input and generating a respective output, all such stages except a first stage receiving an input from a previous stage and each of said stages except a last stage applying its output to a subsequent stage;

an isolation element connecting each of a plurality of electrical contacts of said logic device to the output of a corresponding stage of said shift register so that the logic level on each electrical contact serving as an input is controlled by the output of a corresponding shift register stage and the logic level on each electrical contact serving as an output is controlled by the output of said logic device, whereby said shift register applies a test vector to said logic device without the need to determine whether each electrical contact is serving as an input or an output;

an exclusive OR gate having an output applied to the input of the first stage of said shift register and a plurality of inputs receiving the outputs of a plurality of stages of said shift register and the signals on a plurality of the electrical contacts of said logic device;

timing and control circuit means connected to said shift register for controlling the operation of said shift register and allowing said logic device to respond to a plurality of said test vectors; and

data output means providing an indication of the response of said logic device to a plurality of said test vectors.

2. The testing system of claim 1 further including a multiplexer having an output connected to one input of said exclusive OR gate and a plurality of inputs each connected to one of the electrical contacts of said logic device, said multiplexer being operated by said timing and control circuit means to sequentially apply individual inputs to its output in synchronization with the operation of said shift register thereby allowing a new test vector to be generated after said multiplexer inputs have been shifted into said shift register.

3. The testing system of claim 2 wherein said timing and control circuit means includes means communicating with said logic device for preventing said logic device from responding to the contents of said shift register while said multiplexer inputs are being applied to said exclusive OR gate and the outputs of said exclusive OR gate are being shifted into said shift register, and means for causing said logic device to respond to the contents of said shift register while preventing said shift register from shifting data from one stage to the next.

4. The testing system of claim 2 wherein said multiplexer includes an output shift register operated by said timing and control circuit, the response of said logic device to said test vector being transferred in parallel to respective stages of said output shift register and then being transferred in serial to said exclusive OR gate while the output of said exclusive OR gate is shifted into said first shift register.

5. The testing system of claim 4 wherein said timing and control circuit includes means for causing said logic device to respond to said test vector while preventing said first shift register from shifting data from one stage to the next, and means for preventing said logic device from responding to said test vector while the response of said logic device to said test vector is being transferred in serial to said exclusive OR gate and the output of said exclusive OR gate is being shifted into said first shift register thereby building a subsequent test vector.

6. The testing system of claim 1 further including a counter recording the number of responses of said logic device to said test vector and causing said timing and control circuit means to prevent further shifting of said shift register when the number of responses to said test vectors reaches a predetermined number.

7. The testing system of claim 6 further including output means for recording the test vector stored in said shift register when said logic device has responded to said predetermined number of test vectors.

8. The testing system of claim 7 wherein said output means includes a read shift register having an input connected to the output of the last stage of said first shift register, said read shift register receiving the final test vector from said first shift register in serial and then transferring the contents of said read shift register to the data bus of a processor in parallel.

9. The testing system of claim 1 further including means for controlling the logic level on predetermined electrical contacts of said logic device independently of the outputs of respective shift register stages connected thereto through said isolation elements.

10. The testing system of claim 1 further including a programmer controlling the operation of said system, said programmer including a processor having a data bus and address bus, and where timing and control circuit means further includes a decoder receiving inputs from said address bus and generating control outputs corresponding thereto to control the operation of said shift register.

11. The testing system of claim 1 further including a programmer controlling the operation of said system, said programmer including a processor having a data bus and an address bus, said testing system further including an adaptor that will be connected to other portions of said system and physically receiving said logic device, said adaptor including storage means containing data indicative of the device, said adaptor including storage means containing data indicative of the characteristics of specific logic devices which said adaptor is adapted to receive, said storage means applying said data to the data bus of said programmer as said storage means is addressed by the address bus of said programmer thereby adapting said programmer to the specific logic device being tested.

12. A method of testing a logic device having a plurality of electrical contacts, some of which serve as inputs and some of which serve as outputs, comprising:

(a) applying an original test vector to the electrical contacts of said logic device through respective isolation elements so that the logic levels on contacts serving as outputs are determined by said logic device while the logic levels on contacts serving as inputs are determined by the components of said original test vectory, whereby said original test vector is applied to said logic device without the need to determine if each electrical contact is serving as an input or an output;

(b) allowing said logic device to respond to said test vector;

(c) creating a new test vector as a function of said logic device's response to the test vector being applied;

(d) applying said new test vector to the electrical contacts of said logic device through respective isolation elements so that the logic levels on contacts serving as output are determined by said logic device while the logic levels on contacts serving as inputs ar determined by the components of said new test vector, whereby said new test vector is applied to said logic device without the need to determine if each electrical contact is serving as an input or as an output;

(e) repeating steps (b), (c) and (d) a predetermined number of cycles; and

(f) comparing the new test vector applied to said logic device during the final cycle to the test vector obtained from a logic device that is functioning properly.

13. The testing method of claim 12 wherein said logic device has a plurality of electrical contacts some of which output the logic device's response to said test vector and some of which input the test vector to said logic device, and wherein said new test vector is created by performing an exclusive OR function with the outputs of said logic device in response to at least some of the results obtained from such previously performed exclusive OR functions.

14. The testing method of claim 13 wherein said exclusive OR function is performed with the outputs of said logic device is sequence and said new test vector is created after said exclusive OR function has been performed with predetermined outputs of said logic device.

15. The testing method of claim 12 wherein the logic levels on said contacts are recorded and wherein said new test vector is created by performing an exclusive OR function with each recorded logic level in sequence and with predetermined bits from a multibit word that initially corresponds to the previous test vector, each bit of said word being sequentially shifted to the next bit as a result of each successive exclusive OR function being shifted into the first bit of said word whereby said word corresponds to said new test vector after said exclusive OR function has been performed with predetermined recorded logic levels.

The United States Of America As Represented By The Secretary Of The Navy

Compact electronics test system having user programmable device interfaces and on-board functions adapted for use with multiple devices under test and in various environments including ones in proximity to a radiation field

The United States Of America As Represented By The Secretary Of The Navy

Compact electronics test system having user programmable device interfaces and on-board functions adapted for use with multiple devices under test and in various environments including ones in proximity to a radiation field